This invention relates to high resistance electrostatic shielding between elements of a deflection yoke and between the deflection yoke and the electron gun elements within a vacuum tube whose electron beam is being deflected.
In a television camera, a deflection yoke including deflection windings is used in combination with each pickup tube to repetitively deflect an electron beam across the surface of a photosensitive target (or conductor). Similarly, in television receivers, a deflection yoke is used in combination with a cathode ray tube to repetitively deflect an electron beam (or beams) across a photoemissive surface.
It is undesirable to have capacitive coupling between the deflection windings and the electron gun elements of the camera pickup tube because, in spite of the presence of by-pass capacitors connecting the electron gun elements to ground, the large voltage pulses associated with the deflection windings (especially the line-rate pulses) will be coupled to, and will produce small voltages on the electron gun elements. Capacitance between the electron gun elements and the target element will couple those voltages to the target element where they combine with the video target signal. These extraneous pulses could, for example, saturate or over-drive the associated video amplifiers. However, if the pickup tube is electrostatically shielded from the yoke, capacitive coupling between the deflection windings and the electron gun is reduced.
It is also undesirable to have capacitive coupling between the deflection windings and the electron gun elements of a cathode ray tube.
Accordingly, it is common practice to interpose an electrostatic shield of conductive, low permeability material between the deflection windings and the camera or cathode ray tube to prevent an electric field from being established by the deflection yoke near the tube elements or in the region through which the electron beam flows, thereby reducing the capacitive coupling.
It is undesirable to have capacitive coupling between the line-rate windings and the field-rate windings of a deflection yoke because a line-rate component in the field-rate deflection current would produce distortion in the scanning raster. Accordingly, it is common practice to interpose an electrostatic shield of conductive, low permeability material between the line-rate windings and the field-rate windings, thereby reducing the capacitive coupling.
Deflection of an electron beam is accomplished by the magnetic field of the deflection windings. The low-frequency components of the magnetic field pass undistorted through such a low permeability shield, and the beam deflection is not hindered. High-frequency components of the magnetic field, however, cause an induced eddy current to flow through the conductive electrostatic shield. The current flow in the conductive electrostatic shield in turn generates a magnetic field tending to cancel the original field by which the current flow was established. Thus, a conductive electrostatic shield which permits significant eddy current flow (e.g., a continuous sheet or film of aluminum or graphite) would also act to shield the electron beam from high-frequency magnetic deflection components, and would distort the magnetic field.
Eddy currents due to high-frequency components of the magnetic deflection field occurring during the line-rate flyback or retrace interval cause a power loss in such a shield. The result of the power loss is undesired nonlinear deflection versus time in the interval immediately following retrace, in spite of current in the line-rate deflection windings which changes linearly versus time during that interval. This results in an apparent picture expansion on the left side of the raster. In addition, the changes in scanning velocity during that interval cause undesired modulation of the video target signal current.
Efforts to reduce the shielding and power loss effects of the electrostatic shield on the magnetic deflection components have in the past been directed to interrupting the flow of eddy currents in the electrostatic shield by introducing gaps of various sorts in the conductive shield. Such an arrangement is shown in U.S. Pat. No. 3,601,648 issued Aug. 24, 1971 to Uno. The reduction in electrostatic shielding occasioned by these gaps has been corrected by overlapping the shields, as described in U.S. Pat. No. 2,490,731 issued Dec. 6, 1949 to Goodale, et al. Such arrangements are complex and expensive. It is desirable to have an electrostatic shield which is continuous and without gaps, and which provides for low attenuation of high-frequency components of the magnetic deflection fields and low power losses.